Liquefaction resistance of granular mixes based on contact density and energy considerations
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Soil liquefaction is a process involving gradual loss of granular contact-stress in cohesionless non-plastic soils, leading to the loss of strength of the soil structure during undrained loading. During this process, energy is continuously lost mainly along frictional contacts. This thesis focuses on (i) examining the role of intergrain contact density and confining stress on steady state strength and cyclic resistance of clean sands and non-plastic silty soils, (ii) formulating an expression for frictional energy loss in soils, and (iii) development of a simple liquefaction evaluation approach based on energy considerations. Intergrain contact density indices in terms of global void ratio and silt content have been proposed. An extensive experimental study was conducted involving undrained monotonic and cyclic triaxial tests on three different sands and their sand-silt mixes to acquire valuable data for evaluations. The applicability of proposed contact indices to characterize steady state strength and cyclic resistance of granular mixes is analyzed experimentally. The results show that at the same initial confinement, the steady state strength of sands and silty soils is a function of contact density. The steady state strength of silty soils depends on both contact density and initial effective confinement. For all the tested soils, cyclic resistance, which is measured in terms of both number of equal stress cycles required to cause liquefaction (N L ) and dissipated energy up to liquefaction (E L ) increases log-linearly with intergrain contact density. E L depends on initial confinement, and increases linearly with increasing initial effective stress. Theoretical expressions are developed for frictional energy loss (W) during cyclic loading. The mobilization of contact friction and the occurrence of slip along the contact surfaces in a soil medium are studied and incorporated into the theoretical development. The cyclic triaxial test data are used to evaluate the developed expression for W. For each cyclic test, the dissipated energy (E) is calculated and compared with corresponding computed W. E and W shows 1:1 relationship ensuring the validity of the expression. Based on above developments and understanding, a numerical simulation model using finite difference method is developed to simulate energy dissipation, pore pressure generation, pore pressure dissipation, and densification in a given soil profile for a given earthquake motion. Utilizing the above simulation model, a simple energy based liquefaction evaluation procedure is developed. The procedure is verified using a centrifuge model test data.